1. Field of the Invention
The present invention relates to a method for synchronizing the time of peripheral nodes in wireless communication networks. More particularly, the present invention relates to synchronizing the time of nodes, such as peripheral nodes, which have no GPS (Global Positioning System) receiver, by using GPS information of a node having a GPS receiver in a communication system.
2. Description of the Related Art
A GPS service is generally provided in the following manner: GPS signals are received from at least three GPS satellites, and accurate time and distance are measured from them by a triangulation method. In other words, based on the triangulation of three different distances, the user is provided with information regarding the accurate current position so that he/she can recognize his/her position. The GPS service has also been extended to a navigation service, which uses additional data (e.g. maps, traffic information) so that, when the user selects a specific destination, he/she is provided with specific information regarding the roads leading from the current position to the destination, which sometimes even includes commands to turn or exit onto intersecting roadways, and other available traffic services, etc. The GPS is also used in other fields including geodetic survey, measurement, military purposes, aviation control, etc.
The synchronization of systems or networks is a crucial factor in wireless communication systems. A typical example of current methods for synchronizing wireless communication networks uses GPS satellites and commonly adopts point-to-point topology between receivers and GPS satellites to receive GPS signals.
FIG. 1 is a block diagram showing the construction of a conventional GPS receiver that uses information from GPS satellites for time synchronization. The GPS receiver uses inputted 8 KHz signals, which are synchronized with GPS signals or GPS 1PPS (Pulse Per Second), as the reference to provide the system with 10 MHz, PP2S (Pulse Per 2 Second), and 1PPS signals that are synchronized therewith.
Still referring to FIG. 1, the GPS receiver 10 includes an antenna interface 110, an FPGA (Field-Programmable Gate Array) 120, a GPS reception unit 130 (hereafter “GPS receiver”), a CPU 140, an oscillator 150, and an input/output unit 160.
The antenna interface 110 is adapted to receive L1 signals from a GPS reception antenna and supply 1PPS signals synchronized with UTC (Coordinated Universal Time). The antenna interface 110 also checks the condition of physical connection with the GPS reception antenna and reports the check result to the system.
The FPGA 120 includes an alarm detector 121 for determining whether or not each Voltage Controlled Oscillator (VCO) inside the GPS receiver 10 has provided an output, whether or not the power is functioning normally, etc. and reporting the result to the CPU 140; a multiplexer 125 for receiving external 1PPS signals and 8 KHz signals synchronized with GPS 1PPS; a phase error detector 122 for receiving an output of the multiplexer 125, which is selected based on a selection control signal, and checking the phase error of received signals; a discrete I/O interface 123 adapted for input/output of signals among a CLK and timing generator 124, the phase error detector 122, and the alarm detector 121; and a CLK and timing generator 124 for generating 1PPS and PP2S output signals required by the system by using GPS or synchronized 10 MHz clocks.
The GPS receiver 130 processes GPS signals received by the antenna interface 110 and provides GPS 1PPS signals.
The CPU 140 is responsible for controlling respective components of the GPS receiver 10 during a GPS reception operation, evaluating an alarm reported by the alarm detector 121, and reporting the current reception condition of the GPS receiver 10 to the system. The current reception condition reported includes at lest one of a FF (Function Failure) condition, a PF (Power Failure) condition, a normal condition, an abnormal condition, and a holdover condition.
Still referring to FIG. 1, the oscillator 150 consists of an OCXO (Oven Controlled X-tal (crystal) Oscillator) or a TCXO (Temperature-Compensated X-tal (crystal) Oscillator), and provides output signals having a mechanically or physically stable oscillation frequency.
The basis of operation of the OCXO is that the crystal is sensitive to changes in temperature. In particular, an oven is used to maintain the temperature around the crystal at a constant level so that no error occurs. The OCXO has the highest level of precision among crystal-applied products. However, the OCXO is bulky and uses various levels of power (12V, 24V, 30V) for operation, and thus the OCXO typically finds use for repeaters or for military purposes (e.g. missiles, satellites) than for personal portable communication.
On the other hand, the TCXO is less expensive than the OXCO, and is commonly used for conventional GPS receivers. The TCXO adopts a temperature compensation circuit, a thermistor, and a VCO (Voltage Controlled Oscillator) to improve the operating performance of the crystal. The temperature compensation circuit limits the change of the TCXO output frequency as the operating temperature varies. The thermistor reduces the oscillation frequency error of the oscillator, which depends on temperature. The VCO has a high level of frequency stability against temperature changes ranging from a number of MHz to tens of MHz, and is widely used as the reference frequency source.
The input/output unit 160 shown in FIG. 1 provides the user with a UART (Universal Asynchronous Receiver/Transmitter) port comprising a debug port and a TOD (Time Of Day) port. The input/output unit 160 uses the TOD port to enable real-time monitoring of the current TOD data and to provide remote control and download functions.
Based on a predetermined reference (e.g. midnight, Jan. 6, 1980), the TOD port counts the currently received 1PPS from the first GPS 1PPS to provide accurate time information. The 1PPS comprises an accurate timing signal, with which each node synchronizes all clocks used in the system.
Conventional methods for synchronization of wireless communication networks employ the above-mentioned GPS receivers to receive GPS information from GPS satellites and conduct synchronization. However, these methods have a problem in that, if GPS information is not properly received from GPS satellites (e.g. in downtown areas with many skyscrapers or obstacles, indoor environments where GPS signals are hardly received), the system cannot be synchronized as desired. In systems which may sometimes include navigation directions, the lack of synchronization can have significant consequences on the system.